The present invention relates to an active infrared sensor utilized in a security system, etc. In particular, the present invention concerns a multi-stage active infrared sensor having a plurality of light emitting means and a plurality of light receiving means which are opposed one by one to the light emitting means across a watch area, and intends to improve reliability in the detection operation of this sensor.
In an example of the related art, Japanese Patent Laid-open Publication No. H8-171679 teaches an active infrared sensor which is applied to a security system in order to detect intrusion of a person into a watch area. This type of sensor is generally equipped with a light emitter containing a light emitting element and a light receiver containing a light receiving element. With the light emitter and the light receiver being opposed across a watch area, the light emitter projects an infrared beam toward the light receiver. If the infrared beam directed from the light emitter to the light receiver is interrupted by an intruder and thus the amount of beam received by the light receiving element changes, the sensor activates a security camera or reports to a security company.
As one of this type of sensors, there is also known a multistage active infrared sensor (also called a multi-beam sensor) having a plurality of light emitters and a plurality of light receivers which are opposed one by one to the light emitters across a watch area (e.g. Japanese Patent Laid-open Publication No. H9-297184).
In the case of the multistage active infrared sensor, an infrared beam emitted by each light emitter radiates in such a wide angle that the infrared beam may be received not only by an opposite light receiver but also by another irrelevant light receiver (which is called mutual interference between infrared beams) In particular, where the light emitters and the light receivers are spaced far apart (e.g. across a watch area with a width of about 100 meters), mutual interference of infrared beams tends to occur frequently.
Under the mutual interference between infrared beams, even when an intruder partially blocks the infrared beams emitted across a watch area, each light receiver still receives infrared beams from the opposite light emitter and another non-opposite light emitter. Therefore, the light receivers may not be able to detect the partial interruption of the infrared beams, and may eventually fail to give an alarm.
In an attempt to solve the undesirable situation caused by mutual interference between infrared beams, each light emitter is designed to emit an infrared beam at a different pulse frequency. In this case, the opposed light emitter and light receiver can operate as an independent sensor unit. When the light receiver receives an infrared beam from a non-opposite light emitter, the received signal is cancelled.
Nevertheless, in the case where each light emitter is designed to emit an infrared beam at a different pulse frequency, each light receiver has to be fitted with a high-performance filter in accordance with the respective channel (frequency). However, if the light emitters and the light receivers are disposed at a short distance, each light receiver receives an excessive quantity of infrared beams beyond the capacity of the filter. Hence, this attempt cannot eliminate mutual interference between infrared beams completely. Besides, mutual interference of infrared beams can occur for other reasons, for example, due to the installation environment of the sensor or worker's error in adjustment or setting. Further, addition of the filter raises the cost of the sensor.